(1) Field of the Invention
The present invention relates to a wiring board such as a printed wiring board etc., and in particular relates to a wiring pattern which alleviates board""s warping due to thermal expansion.
(2) Description of the Prior Art
A printed wiring board is composed of an electrical insulating substrate (e.g., glass-epoxy baseboard) and a wiring pattern of interconnections of copper as a conductor on the surface thereof or in an inner layer thereof. The wiring pattern may be formed on one or both sides of the board. Now, referring to FIG. 1, a double-sided wiring board having a pattern of interconnections on both sides of the substrate will be described as the prior art.
In order to lower the conductor""s resistance, a solid wiring pattern of interconnections for power source lines and/or ground (GND) lines 41 which have much greater widths compared to other typical signal lines is mainly formed on one side of a glass-epoxy baseboard 40 in the lateral direction (the direction parallel to the document surface). On the opposite side, typical signal lines 42 which need less current-carrying capacity are formed mainly in the longitudinal direction (the direction perpendicular to the document). Though the figure sectionally shows a power source (ground) line 41 laid out in the longitudinal direction in order to represent the fact that power source (ground) line 41 is wider than signal lines 42 on the opposite side, a power source (ground) line 41 is generally formed in the lateral direction. Here, it is understood that the relationship of the orientations of the above interconnections may be reversed.
When ICs and/or other electric components are mounted on such a printed wiring board, the board is thermally treated by reflow soldering etc. (at 200 to 260xc2x0 C.). As a result, warping occurs in the board due to difference in thermal expansion coefficient between the insulating substrate and wiring pattern (conductor), causing terminal floating (of the electric components), solder cracking and the like. This phenomenon is more likely to arise as the board becomes thinner. Also, when a broad solid conducting pattern such as a power source line, ground line, etc., is formed on one side only, the phenomenon more often occurs, greatly contributing to mounting failures.
For countermeasures against these problems, there have been disclosed Japanese Patent Application Laid-Open Hei 4 No.188886, Japanese Patent Application Laid-Open Sho 62 No.155586. In the former case, a wiring pattern of interconnections is formed on one side of an insulating substrate while a dummy pattern having interconnections arranged in the same direction as above is formed on the other side so as to prevent the board from warping due to difference in thermal expansion coefficient. In the latter case, a solid conductor pattern having an area greater than 50% of the total board area is formed on the undersurface of the insulating substrate while a wiring pattern having conducting paths of large widths or island-like conductor pads is formed on the obverse surface of the insulating substrate, directly opposite the solid conductor pattern, whereby warping of the board is prevented. Both of these countermeasures attempt to make the areas of copper foil on both sides of the insulating substrate as equal to each other as possible to reduce warping attributed to difference in thermal expansion coefficient between the copper wiring and the insulating substrate.
However, in a method as above, it is necessary to align the directions of thermal expansion of, and equal the areas of, the conductor patterns on both sides of the insulating substrate. So this will restrict the conductor pattern formed on the undersurface of the insulating substrate. As a result, if there is no margin to meet these restrictions, it is impossible to use the method.
The present invention has been devised in view of the above problems in the prior art, and it is therefore an object of the present invention to provide a wiring board which can be prevented from warping due to difference in thermal expansion with a markedly simple configuration.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, a wiring board for mounting and connection of electric components comprises: an insulating substrate; and a wiring pattern of conducting paths formed on at least one side of the insulating substrate, and is characterized in that, among the conducting paths formed on the insulating substrate, all or part thereof which needs a predetermined current-carrying capacity and hence would have been formed with broad conducting paths is divided into multiple fine-line interconnections.
In accordance with the second aspect of the present invention, the wiring board having the above first feature is characterized in that a short-circuiting joint is formed to electrically join the divided, multiple fine-line interconnections.
In accordance with the third aspect of the present invention, a wiring board for mounting and connection of electric components comprises: an insulating substrate; and a wiring pattern of conducting paths formed on at least one side of the insulating substrate, and is characterized in that, among the conducting paths formed on the insulating substrate, all or part thereof which needs a predetermined current-carrying capacity and hence would have been formed with broad conducting paths is formed in a cranked configuration.
In accordance with the fourth aspect of the present invention, a wiring board for mounting and connection of electric components comprises: an insulating substrate; and a wiring pattern of conducting paths formed on at least one side of the insulating substrate, and is characterized in that, among the conducting paths formed on the insulating substrate, all or part thereof which needs a predetermined current-carrying capacity and would have been formed with broad conducting paths is divided into multiple fine-line interconnections and all or part of the divided multiple fine-line interconnections is arranged in a cranked configuration.
In accordance with the fifth aspect of the present invention, the wiring board having the above fourth feature is characterized in that a short-circuiting joint is formed to electrically join the divided, cranked, multiple fine-line interconnections.
In accordance with the sixth aspect of the present invention, the wiring board having any one of the above first, second, third, fourth and fifth features is characterized in that conducting paths needing a predetermined current-carrying amount are of a power source line or of a ground line.
According to the wiring board of the present invention, since whole or part of the power source lines or ground lines etc., conventionally needing broad conducting paths, is divided into a pattern with multiple fine-line interconnections, it is possible to reduce the stress arising due to difference in thermal expansion coefficient, and hence reduce the warping of the board due to difference in thermal expansion coefficient between the insulating substrate and the wiring pattern. Accordingly, it is possible to prevent terminal floating and/or solder cracking which would be caused by warping of the board, and hence eliminate connection (mounting) failures.
Further, according to the wiring board of the present invention, since whole or part of power source lines or ground lines, etc., conventionally needing broad conducting paths, is formed in a cranked configuration, it is possible to break up or offset the stress due to difference in thermal expansion coefficient, and hence reduce the warping of the board due to difference in thermal expansion coefficient between the insulating substrate and the wiring pattern. Accordingly, it is possible to prevent terminal floating and/or solder cracking which would be caused by warping of the board, thus making it possible to eliminate connection (mounting) failures.